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HUMAN BRAIN |
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Basic Brain Anatomy
The human brain, weighing approximately three pounds or roughly two percent of body weight, can be thought of as the sacred vessel that contains and reproduces the world around us. All of our conscious and unconsciousness perceptions are filtered, altered, analyzed and organized by a gigantic symphony of neuronal players and then distributed to other body organs via nerve impulses and biochemical messengers. Our entire experiential universe is contained within the substance of the brain's matter. However, one perplexing question is generated by the study of the brain. What happens after the brain ceases to function and has surrendered to the dreaded ghost of impermanence? Is there something within the brain that lives on as the soul? Is there truly a "seat of consciousness" within the brain?
Prior to a brief discourse on the origin of consciousness and the seat of the soul, it would behoove us to review some basic brain anatomy.
The central nervous system (CNS) consists of the brain and spinal cord comprised of 200 billion neurons. In the developing embryo, a neural groove soon differentiates into a neural tube and at the cephalic or "head end", the brain forms as the prosencephalon (forebrain) which is further subdivided into the diencephalon and the telencephalon. When we think of the brain, we often picture two roughly equal-sized hemispheres, the inferior and posterior cerebellum or "small brain", and the midline brain stem, which continues distally as the spinal cord. The forebrain is the most rostral, ( i.e. closest to the nasal passages), portion of the three primary brain vesicles of the embryonic neural tube. The prosencephalon further divides into the diencephalon and the telencephalon. The second major brain vesicle is the mesencephalon or midbrain. The third major division is the rombencephalon (hindbrain). The neocortex is a structure belonging to the forebrain. In an embryo of three months gestation, approximately 250,000 neurons per minute are formed. At birth, almost the entire adult complement of neurons (30 billion or more) has been formed. At six years of age, the brain weight is one-half of the adult weight. At age 10, the brain weight is nearly the same as the adult brain and reaches its maximum weight at age 20. This increase in brain size is due to the rapid formation of new connections, myelination or "insulation" of the axonal fibers and growth of glial (supporting) cells.
The two great systems of the CNS are the motor and sensory systems. Motor, or muscular control, requires many connections from the cortex of the brain, to the basal ganglia, deep brain structures of grey matter, and to the cerebellum (for coordination) and eventually to motor neurons in the spinal cord and finally to the muscular system. Incoming sensory signals (touch, vibration, pain, position sense, temperature, etc.) return via the spinal cord to the thalamus, the sensory relay station and then to the cortex. This is, of course, a simplification of a vast interconnected network of neurons. Approximately 75% of the brain's neurons are located in the cerebral cortex with its billions of cells and over one million billion (ten to the ninth power) of connections. It has been stated by Gerald Edelman that this number exceeds the number of known particles in the universe (ten to the 80th power) with the number of possible neuronal circuits being ten to the millionth power! The cerebral hemispheres are further subdivided into various lobes, and designated frontal, temporal, parietal and occipital (Fig. 1).
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| Figure 1 |
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These divisions (Figure 2) roughly correspond to anatomical dividing lines of various named sulci and gyri. Simply put, one could consider the frontal lobes are for personality however within the frontal lobes are connections concerning speech, functions of the mouth and pharynx, regions concerned with articulation and deglutition (the process of swallowing), portions of respiratory and circulatory control, intelligence, planning, reasoning and thinking. The anterior region, the pre-frontal cortex, is a location where intelligence was once thought to be located however, with destruction of the pre-frontal area, there is no significant observable decrease in intellectual performance. Electrical stimulation of the pre-frontal area produces no motor movements and thus is called inexcitable.
It is useful to think of the parietal lobes in terms of spatial orientation, speech, cognition, integration of auditory, tactile and visual memories along with interconnections to the temporal and occipital lobes both intrahemispheric and, via the corpus callosum, with the contralateral hemisphere. Various disorders of language, various apraxias (disorders of movement), visual field cuts and patterns of body neglect are seen with damage to the parietal lobes.
The temporal lobes contain a preponderance of fibers involved with facial recognition. Other functions include audition and balance, a portion of the pathways for vision (i.e. temporal lobe lesions produce superior contralateral quadrantic visual field cuts) and a portion of the olfactory apparatus for the sense of smell. Portions of the limbic system, which plays a major role in memory, are located within the temporal lobe.
The occipital lobes are concerned with our visual apparatus and the pathways for perception and recognition. The figure below shows further divisions based upon the function of various regions of the brain cortex:
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| Figure 2 |
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The frontal lobe lies anterior to the fissure of Rolando (central sulcus) and superior to the fissure of Sylvius (Figure 3).
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| Figure 3 |
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